As piezoelectric ceramic materials containing Pb, Zr, Ti and O, for example, known are two-component materials such as PbZrO.sub.3 -PbTiO.sub.3, three-component materials such as Pb(Sb.sub.0.5 Sn.sub.0.5)O.sub.3 -PbZrO.sub.3 -PbTiO.sub.3, and composite materials prepared by adding various minor additives to those materials.
Having good piezoelectric characteristics and capable of being processed with ease and being easily produced in large quantities in mass-production systems, these piezoelectric ceramic materials have many applications in various fields of filters, actuators, piezoelectric transformers, piezoelectric sensors, etc.
Recently, various monolithic piezoelectric structures are being used in those application devices. Monolithic piezoelectric structures comprise a plurality of piezoelectric ceramic layers and a plurality of electrodes sandwiched between the adjacent piezoelectric ceramic layers, in which the layers and the electrodes are integrally laminated. To produce such monolithic piezoelectric structures, the piezoelectric ceramic materials and electrode materials for those must be co-fired.
However, the conventional piezoelectric ceramic materials such as those mentioned above generally have a high sintering temperature of about 1200.degree. C. or so. Therefore, expensive electrode materials such as Pt must be used along with those piezoelectric ceramic materials to obtain the intended monolithic piezoelectric structures. If, on the other hand, inexpensive electrode materials such as AgPd are used, some sintering aids such as PbO and SiO.sub.2 must be added to the piezoelectric ceramic materials to thereby lower their sintering temperature.
The use of expensive electrode materials is unfavorable, since the costs of the materials for piezoelectric application devices increase. On the other hand, the addition of sintering aids such as PbO and SiO.sub.2 is also unfavorable, as they worsen the piezoelectric characteristics of the piezoelectric ceramic materials containing such sintering aids. In order to prevent the worsening of the piezoelectric characteristics, some measures may be taken to optimize the piezoelectric ceramic compositions themselves or to optimize the amount of the additive to be added to the piezoelectric ceramic compositions, thereby improving their piezoelectric characteristics. These measures, however, are problematic in that novel piezoelectric ceramic materials that are exclusively applicable to the production of monolithic piezoelectric structures must be developed and that development takes a lot of time and is expensive.